1. Field of the Invention
The present invention relates to an air to fuel ratio control system for a gasoline engine having a fixed venturi type carburetor.
2. Description of the Prior Art
The gasoline engine for an automobile requires fast response of the air to fuel ratio control system since operating condition of the engine frequently changes. A prior method for controlling an air to fuel ratio by sensing a change in composition of exhaust gas used in the air to fuel ratio control system has a drawback of slow response. Namely, since a certain period of time is taken before air-fuel mixture supplied from the carburetor to the engine passes through an intake pipe of the engine, a combustion chamber and an exhaust pipe and finally reaches an exhaust gas sensor, the rapid change of operating condition cannot be followed. If a gain of the control system is increased to overcome the above drawback, the air to fuel ratio control system will hunt and a proper control will not be attained. The closed loop control for the air to fuel ratio using the exhaust gas sensor is effective to the air to fuel ratio control in which the air to fuel ratio is corrected for slow change of environment such as secular change of metering of fuel for the carburetor, level change of ground or temperature change, but the development of control method having higher response and stability has been desired.
An air to fuel ratio control method which does not use the exhaust gas sensor is disclosed, for example, in the U.S. Pat. No. 3,750,632. In the method disclosed therein, the amount of suction air is measured by a heat radiation type or moving pressure plate type air flow meter to produce an electrical signal and based on this signal a desired flow rate of fuel is calculated by an electric circuit. On the other hand, an actual flow rate of fuel is sensed by a fuel flow meter, and the flow rate of fuel is controlled based on a difference between the desired flow rate and the actual flow rate of fuel. It would be expected that this control method attains better control response than the air to fuel control method using the exhaust gas sensor, but the signal produced from the air flow meter is not a linear function of the amount of intaking air while the signal from the fuel flow meter is proportional to the actual flow rate of fuel. Because of the use of different type of measurements, a complex linearizer is necessary in calculating the desired flow rate of fuel based on the intake air amount signal. Accordingly, the air to fuel ratio control system disclosed in the above U.S. Patent will be complex and expensive.